Soybeans are called the meat of the field. They are rich in protein having an amino acid composition similar to that of animal protein, and contain good quality protein comparable to that of meat and eggs. Soybeans are also rich in oil (fat), and 50% or more thereof is linoleic acid, which functions to decrease blood cholesterol. Soybeans are known to be effective in preventing adult diseases, in particular, high blood pressure. Further, soybeans contain lecithin, and are known to be effective in preventing dementia by acting on brain cells.
Still further, soybeans contain nutrients such as vitamins, including vitamin B1, vitamin B2, vitamin E, vitamin K, etc.; calcium; potassium; fiber; etc. These nutrients are known to be effective in relieving fatigue and preventing aging, constipation, etc.
It has been reported that soybeans also contain various isoflavones having a mild female hormone-like action, and are effective in preventing or ameliorating menopausal symptoms, osteoporosis, etc.
As described above, soybeans contain various useful nutrients, and have been drawing attention as a non-meat food material with a high nutritional value and a good nutritional balance. In recent years, a technique for producing soybean powder having excellent texture and containing substantially all of the soybean nutrients has been developed (for example, Patent Literature 1 and Patent Literature 2). In response to the recent health consciousness of consumers, various food products and beverages that use the soybean powder have been attracting attention.
At the same time, soybeans contain urease, which is an enzyme that hydrolyzes urea and produces ammonia. When urease is taken into the body, it comes into contact with urea that exists in the intestinal tract, and ammonia is thereby formed. The ammonia thus formed is absorbed in the body, and the greatest part thereof is transformed into urea in the liver. However, patients who have liver disorders, such as cirrhosis of the liver, exhibit lowered ureagenesis in the liver. This results in the accumulation of ammonia in the body and the elevation of the ammonia levels in blood. The elevation of ammonia levels in the blood is believed to be a main cause of brain disorders, such as hepatic encephalopathy and hepatic comas. Urease is also known to facilitate the colonization of Helicobacter Pylori, which is a pathogenic bacterium that causes gastritis and gastric ulcers. Furthermore, the ammonia formed by urease is toxic to enterobacilli, resulting in the aggravation of intestinal bacterial flora.
In China, a fatal accident was caused by a trypsin inhibitor contained in soybean. In response, national standards were established for soybean powder and soy milk. Since the trypsin inhibitor has almost the same enzyme inactivation temperature range as urease, the urease activity value is used as an indicator in the Chinese national standards that regulate the safety of soybean powder and soy milk.
As described above, it is important to prevent an excessive intake of a trypsin inhibitor and urease in order to maintain a healthy condition and prevent disease. Therefore, it is believed that food materials that are taken daily, such as soybeans, should desirably have a reduced urease activity.
The soybean powder disclosed in Patent Literature 1 is excellent in nutritional value, texture, flavor, and the like, and is free from any problems in terms of safety. However, in order to favorably respond to the recent and increasing health consciousness of consumers, the provision of a higher value-added soybean material is in demand.